- Implement a first cut for const String.env in the VM to allow

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
===================================================================
--- sdk/lib/core/string.dart	(revision 28841)
+++ sdk/lib/core/string.dart	(working copy)
@@ -125,6 +125,14 @@
   }
 
   /**
+   * Returns the string value of the given environment variable [name].
+   * The [defaultValue] is returned if [name] is not present in the environment
+   * or if its value is not a string.
+   */
+  external const factory String.fromEnvironment(String name,
+                                                {String defaultValue: ''});
+
+  /**
    * Gets the character (as a single-code-unit [String]) at the given [index].
    *
    * The returned string represents exactly one UTF-16 code unit, which may be